What time is his first nap the day before? That can have an influence on their wake up time the next morning for some reason - if their first nap is too early then they can see it as an extension to their night sleep. Tricky one to fix though if he has been up so early then he will need to get back to bed earlier!
Is he on solids yet? Could be time for that if not?
It can frustrating especially when you know he can sleep for 12 hours!!